[20.15] Polarization of Cosmic Dust in Various Environments

K. Lumme (Observatory, University of Helsinki)

Cosmic dust covers the surfaces of atmosphereless bodies, is
the reason for zodiacal light, and is an important
incredient in the cometary comae. The extensive data sets
show systematical differences in linear polarization between
these three different environments. The polarization maximum
is inversely proportional to the wavelength for the
regoliths and interplanetary dust while the opposite is true
for the comets. The phase angle of the minimum of the
negative polarization branch for the regoliths is
systematically smaller than that of the comets.

To understand these observational effects we have been
collecting and creating various light scattering codes for
this work. Multiwavelength data sets of comets range from
the UV-band to the J-band. We can show that if the cometary
particles are (as a first approximation) elongated cylinders
with the aspect ratio l/d about three and a mean volume
equivalent sphere radius about 0.4 \mu m and obey power
law size distribution with \gamma about -3 all the data
can be fitted with the rms error about 1%. The refractive
index varies with the wavelength about its mean value
1.6+i*0.1 explaining the color dependency of polarization.
We cannot yet claim that this model is unique.

To explain the reversed polarization maximum vs. wavelength
and the position of the minimum polarization of the
planetary regoliths we invoke two mechanisms. Because the
regoliths are almost infinite layers in terms of the optical
thickness both multiple scattering and coherent
backscattering become important. The first of these can
nicely explain the wavelength dependency because the single
scattering albedo decreases with the wavelength. As recently
shown by Mishchenko (2000) coherent backscattering causes a
strongly asymmetric form for the negative branch with the
minimum close to the exact opposition which could explain
the observed effect described above.